Process for the preparation of 3-methylpiperidine and 3-methylpyridine by catalytic cyclization of 2-methyl-1, 5-diaminopentane

ABSTRACT

In a process for preparing 3-methylpiperidine or 3-methylpyridine from 2-methyl-1,5-diaminopentane in the gaseous phase, the initial product is made to flow over catalysts. In the first step, 3-methylpiperidine is produced, and if required 3-methylpyridine is produced in a second step.

This application is a 371 of PCT/EP94/01005, Mar. 30, 1994.

The present invention relates to a process for the preparation of3-methylpyridine (MPI) or 3-methylpyridine (PIC) from2-methyl-1,5-diaminopentane (MPDA).

3-Methylpiperidine is used as a vulcanization accelerator and as anadditive to lubricant oil. 3-Methylpyridine is used both as a solventand as an intermediate in the preparation of nicotinic acid.

PCT application WO 90/00546 discloses the preparation of mixtures of3-methylpiperidine and 3-methylpyridine starting from2-methyl-1,5-diaminopentane by passing the gaseous starting materialover a catalyst comprising metal oxides at 500°-600° C. Preferredcatalysts are copper chromite, molybdenumoxide and vanadium oxide. Thesecatalysts are preferably applied to a support. Depending on the reactiontemperature, the ratio between piperidine and pyridine can be shifted toone or the other side. This patent specification also mentions thepossibility of using acidic oxides, such as SiO₂ or silicon aluminiumoxides, without further additives as catalysts. However, the yieldsachieved in this way are only moderate. No information is given on thecatalyst activity over extended operating times.

U.S. Pat. No. 3,903,079 discloses a process for the cycloammonolysis ofdisubstituted alkanes containing primary am/no and/or hydroxyl groups.The catalyst used is a metal aluminosilicate molecular sieve. Preferredmetals are copper, palladium, manganese, nickel and chromium. Thereaction is carried out in the presence of ammonia. The yields achievedare modest. A yield of 75% is achieved in the preparation of piperidinefrom 1,5-pentanediol.

The object of the present invention is to provide a process for thepreparation of 3-methylpiperidine from 2-methyl-1,5-diaminopentane whichcan be carried out on a commercial scale and achieves high yields. Thecatalyst activity should be maintained over long times. A further objectis to provide a process for the preparation of 3-methylpiperidine byfurther reaction of the 3-methylpiperidine over a dehydrogenationcatalyst.

The object is achieved according to the invention by the processaccording to Patent claim 1.

The term "oxides of Al and/or Si" used in claim 1 is taken to mean theindividual oxides, such as Al₂ O₃, mixed oxides of Al₂ O₃ /SiO₂ andcrystallized compounds thereof, such as aluminium silicates, inparticular zeolites. It is important that they have a predominantlyacidic character and a specific surface area of greater than 40 m² /g.The acidic character arises from the ratio between acidic and basiccenters on the surface, which must, in accordance with the invention, begreater than 2. The acidic centers are determined analytically byirreversible adsorption of NH₃ at 80° C., and the basic centres byirreversible adsorption of CO₂ at 80° C. Preferred catalysts for thenovel process are activated Al₂ O₃, mixed oxides of Al₂ O₃ /SiO₂, orzeolites. Zeolites are crystalline natural or synthetic aluminiumsilicates which have a highly ordered structure with a rigidthree-dimensional network of SiO₄ and AlO₄ tetrahedra connected bycommon oxygen atoms. The ratio between the number of Si and Al atoms andoxygen is 1:2. The electrovalence of the aluminium-containing tetrahedrais compensated by inclusion of cations in the crystal, for examplealkali metal or hydrogen ions. Cation exchange is possible. The spacesbetween the tetrahedra are occupied by water molecules beforedehydration by drying or calcination.

If the zeolite, owing to its preparation method, is not in thecatalytically active, acidic H form, but instead, for example, in the Naform, it can be converted fully or partially into the desired H form byion exchange, for example with ammonium ions, followed by calcination orby treatment with acids.

The catalysts are preferably employed as fixed-bed catalysts, and thestarting material is expediently passed through the catalyst usinghydrogen or an inert gas, such as nitrogen, as carrier gas.

The reaction temperature is set at 300°-400° C., preferably at 305°-375°C. The pressure is 0-10 bar, preferably 0-5 bar above atmospheric.

A measure of the flow rate over catalysts is the mass hourly spacevelocity (MHSV). In the present case, an MHSV of 2.1-4.2 g of startingmaterial per g of catalyst and per hour is advantageously maintained.The vapor-form starting material can be diluted, preferably with N₂ orH₂.

3-Methylpiperidine can be converted into 3-picoline by knowndehydrogenation processes. The 3-methylpiperidine stream produced by theprocess of the invention can be passed directly over a dehydrogenationcatalyst, so that the dehydrogenation takes place immediately after thecyclization. This is possible because the 3-methylpiperidine is producedin unusually high purity and in particular now contains virtually noMPDA, which has been found greatly to impair the activity ofdehydrogenation catalysts.

The dehydrogenation catalysts used are preferably noble metals, such as,for example, Pd or Pt, on a support. Particularly advantageousdehydrogenation catalysts have been found to be those obtainable fromamorphous silicon aluminium oxides by ion exchange with solublepalladium complexes, such as Pd(NH₃)₄ !Cl₂. The amorphous siliconaluminium oxides are advantageously first dewatered and charged withammonia. The ion exchange with the soluble palladium complex can takeplace by suspension of the amorphous oxide in a solution of the complex.Alternatively, a solution of the complex can be passed through a packingof the amorphous oxide, but, in contrast to the former method, uniformloading can only be achieved by complete exchange.

The above methods also allow palladium contents of up to 5% by weight ormore to be achieved in one step using relatively dilute solutions, forexample 0.01 mol/l of Pd(NH₃)₄ !Cl₂.

The reaction temperature during the dehydrogenation is preferably220°-400° C. In one embodiment, the cyclization catalyst is applieddirectly to the dehydrogenation catalyst bed, and the2-methyl-1,5-diaminopentane is passed in from above. In a preferredembodiment, the catalysts are introduced into separate reactors. Thisallows independent temperature control and, if desired, independentcatalyst regeneration.

The examples below illustrate the way in which the novel process iscarried out. The pressures given in the examples are not absolutepressures, but excess pressures above atmospheric.

EXAMPLES 1-11

The examples shown in Table 1 below for the cyclization ofmethyldiaminopentane (MPDA) to methylpiperidine (MPI) were carried outas follows. Examples 1, 2 and 3 are comparative examples (not inaccordance with the invention).

3 g of catalyst (particle size 0.32-1 mm) were introduced into a reactor(.o slashed.13 mm). MPDA was evaporated and passed over the catalystwith a 15 ml/min carrier-gas stream of N₂ at a pressure of 5 bar. Thecatalyst bed was heated in steps, and the reaction was monitored by gaschromatography. The more active the catalyst, the lower the temperaturenecessary for the cyclization of MPDA to MPI. The activity of thecatalysts used can be compared with one another via the temperaturesnecessary for the highest possible MPI yield and taking into account themass hourly space velocity.

The table is supplemented by characterization data on the catalystsused.

                                      TABLE 1                                     __________________________________________________________________________    MPDA to MPI:                                                                                                            Average                                                             BET       particle                                                                           Acidic                                                                              Basic                                T   p   MHSV MPI    surf. area                                                                         Pore vol.                                                                          diameter                                                                           centres (A)                                                                         centres (B)              Example                                                                             Catalyst                                                                             °C.!                                                                       bar!                                                                              g/(g · h)!                                                                % in product!                                                                        m.sup.2 /g!                                                                        cm.sup.3 /g!                                                                       Å!                                                                             μmol/g!                                                                          μmol/g!                                                                         A/B                __________________________________________________________________________    1     Cu chromite                                                                         380 5.0 2.1  13.3   120  0.37 113  203     0   --                 2     ZrO(OH).sub.x                                                                       350 5.0 2.1   6.5   240  0.25  32  150   99.5  1.5                3     SiO.sub.2                                                                           400 5.0 4.2  63.0   347  1.18 125  1400  1000  1.4                4     A1-4405 E                                                                           375 5.0 4.2  90.5   256  0.75 102  no measurement                 5     A1-3996 E                                                                           350 5.0 2.1  89.0   211  0.74 116  165   52.5  3.1                6     K-Y   360 5.0 4.2  94.9                  800   >150  <5.3               7     H-Y   320 5.0 4.2  97.6                  2200   60   37.7               8     H-Y   325 5.0 4.2  97.6                                                 9     H-Y   325 1.8 4.2  98.7                                                 10    Si-235-1 T                                                                          320 5.0 4.2  99.3   376  0.5   47  156    1.6  97.5               11    H-ZSM-5                                                                             305 5.0 4.2  99.6                  650   approx.                                                                             7.2                __________________________________________________________________________     Cu chromite: Cu1230 R (Engelhard; 29% of Cu, 32% of Cr, 6% of Ba)             Al4405 E: 97% of Al.sub.2 O.sub.3 - 3% of SiO.sub.2 (Engelhard)               Al3996 E: Al.sub.2 O.sub.3 (Engelhard)                                        KY: zeolite Y, K.sup.+ - exchanged                                            HY: zeolite Y (Degussa)                                                       Si235-1 T: 87% of SiO.sub.2 = 13% of Al.sub.2 O.sub.3 (Engelhard)             HZSM-5: 54.5% of pentasil (Si/Al = 18) + 45.5% of binder                 

EXAMPLE 12

MPDA to 3-picoline:

4 g of a Pd catalyst (1% of Pd/Al₂ O₃) were introduced into a reactor(.o slashed.13 mm) and covered by 3 g of H--ZSM-5. (The startingmaterial was in all cases introduced into the reactor from the top). Theworking conditions were as follows: temperature 305°-320° C., 15 ml/minof N₂, pressure 5 bar. Yields of up to 97% of 3-picoline were obtainedin the temperature range 305°-320° C. and at an MHSV of 0.6 g/(g.h), theonly further product found being 2.9% of MPI. Complete conversion of theMPDA to the desired products thus occurred. No deactivation of thecatalysts was observed over the course of 10 days. It is also possibleto replace N₂ as carrier gas by H₂.

The novel procedure thus results in a significant improvement in theactivity, selectivity and catalyst service life.

EXAMPLE 13

Preparation of 3-picoline using two separate reactors and commercialMPDA (MPDA to 3-picoline in 2 steps with isolation of MPI):

1st Step: 3 g of ZSM-5 in the ammonium form (particle size 0.5-1 mm)were introduced into a reactor (.o slashed.13 mm). MPDA was evaporatedand passed over the catalyst with a 15 ml/min carrier-gas stream of N₂at a pressure of 5 bar and a temperature of 335° C. The MHSV was 4.2 gof MPDA per gram of catalyst per hour. The MPDA used was a commercialproduct obtainable from Du Pont de Nemours under the trade name Dytek A.The experiment ran for 280 hours. Deactivation of the catalyst was notobserved. The product was condensed, and the ammonia formed allowed toescape. The yields of MPI were virtually quantitative (>99.5%).

2nd Step: 10 g of a Pd-MgCl₂ /Al₂ O₃ dehydrogenation catalyst wereintroduced into a reactor (.o slashed.13 mm). The MPI from the previousexperiment was passed in vapour form over the catalyst with a 15 ml/mincarrier-gas stream of N₂ at a pressure of 1 bar and a temperature of280° C. The MBSV was 0.23 g of MPI per gram of catalyst per hour. Theexperiment ran for 190 hours. Deactivation of the catalyst was notobserved. After 190 hours, the following product composition wasdetermined by gas chromatography: 99.3% of 3-picoline, 0.4% of MPI.

EXAMPLE 14

Preparation of 3-picoline using two separate reactors and commercialMPDA (MPDA to 3-picoline in 2 steps without isolation of MPI):

3 g of NH₄ -ZSM-5 (particle size 0.5-1 mm) were introduced into areactor (.o slashed.13 mm). MPDA was evaporated and passed over thecatalyst with a 15 ml/min carrier-gas stream of N₂ at a pressure ofabout 1 bar and a temperature of 320° C. The MHSV was between 1 and 2 gof MPDA per gram of ZSM-5 per hour. The MPDA used was a commercialproduct obtainable from Du Pont de Nemours under the trade name Dytek A.The product from the cyclization reactor was kept in the gas phase andpassed directly to the second reactor. This reactor contained 12 g of adehydrogenation catalyst having the composition Pd+MgCl₂ on an Al₂ O₃support (particle size 0.32-1 mm). The reaction conditions were 280° C.and about 1 bar. The condensate from the dehydrogenation reactor after areaction time of 220 hours contained 99.1% of 3-picoline and 0.9% of MPI(according to gas chromatography). Deactivation of the two catalystsover the reaction time was not observed.

EXAMPLE 15 Comparative Example

Preparation of a 1% Pd/Al₂ O₃ catalyst by impregnation:

6.3 g of Pd(NO₃)₂ hydrate (Heraeus) and 15.3 g of conc. HCl were addedto 540 g of demineralized water. A pH of 0.7 became established. Thissolution was added to 250 g of Al₂ O₃ (Al-4191 E 1/16" from Engelhard)which had been moistened in advance with demineralized water. Theimpregnation time was 3 days. The solution was subsequently decanted offand the catalyst was dried at 150° C. for 20 hours, then calcined at550° C. for 2 hours in an air-circulation oven and granulated, and thesieve fraction from 0.315 to 1 mm was collected.

EXAMPLE 16 Comparative Example

Preparation of a 3% Pd/Al₂ O₃ catalyst by impregnation:

Al₂ O₃ (Al-3996 R from Engelhard) was granulated and the sieve fractionfrom 0.315 to 1 was used. Three impregnation solutions comprising 150 gof demineralized water, 1.8 g of Pd(NO₃)₂ hydrate (Heraeus) and 2.36 gof conc. HCl were prepared. A pH of approx. 0.8 became established. 70 gof the support were impregnated successively for 24 hours in each casewith these three impregnation solutions; after each impregnation step,the catalyst was washed with 100 ml of demineralized water, dried at150° C. for 2 hours in a vacuum oven and calcined at 550° C. for 2 hoursin an air-circulation oven.

EXAMPLE 17 Comparative Example

Preparation of a 4% Pd/Al₂ O₃ catalyst by impregnation:

Two impregnation solutions comprising 150 g of demineralized water, 1.25g of Pd(NO₃)₂ hydrate (Heraesu) and 2.24 g of conc. HCl were prepared. ApH of 0.8 became established. 50 g of the catalyst from Example 2 wereimpregnated successively with these impregnation solutions; after eachstep, the catalyst was washed with 100 ml of demineralized water, driedat 150° C. for 2 hours in a vacuum oven and calcined at 550° C. for 2hours in an air-circulation oven.

EXAMPLE 18

Preparation of a 5% Pd-SiO₂ /Al₂ O₃ catalyst by ion exchange withPd(NH₃)₄ !²⁺. The Si/Al oxide support (13% by weight of Al₂ O₃)(Si-235-1 T from Engelhard) was granulated (0.315-1 mm). 50 g of thegranules were dehydrated for 12 hours at 400° C. in a quartz tube in astream of N₂. Dry ammonia gas (36 g) was passed over the cooled samplefor 1 hour. A 0.01 molar Pd(NH₃)₄ !Cl₂ solution was prepared: 0.375 g ofPdCl₂ were added to 100 ml of 0.84 molar aqueous NH₃ solution, and themixture was stirred at 85° C. for 15 minutes. After cooling, the desiredmolarity was established by addition of water. 20 g of the pretreatedsupport were stirred for 24 hours with 2542 ml of the 0.01 molar Pd saltsolution. The catalyst was subsequently washed 6 times with 500 ml ofdemineralized water in each case and dried at 120° C. for 24 hours. Thecatalyst contained approx. 5% by weight of Pd.

EXAMPLE 19

Preparation of a 5% Pd-SiO₂ /Al₂ O₃ catalyst by ion exchange withPd(NH₃)₄ !²⁺ :

150 g of the Si/Al oxide support (15% by weight of Al₂ O₃) (Si-HP-87-069T from Engelhard) were dehydrated for 12 hours at 400° C. in a quartztube in a stream of N₂. Dry ammonia gas (60 g) was passed over thecooled sample for 1 hour. 70 g of the pretreated support were stirredfor 20 hours with 3720 ml of a 0.01 molar Pd salt solution (prepared asdescribed in Example 18). The catalyst was subsequently washed 6 timeswith 1000 ml of demineralized water and dried at 120° C. for 15 hours.The catalyst contained approx. 5% by weight of Pd.

EXAMPLE 20

Preparation of a 3% Pd-SiO₂ /Al₂ O₃ catalyst by ion exchange withPd(NH₃)₄ !²⁺ :

120 g of the Si/Al oxide support (15% by weight of Al₂ O₃) (Si-HP-87-069T from Engelhard) were dehydrated for 12 hours at 400° C. in a quartztube in a stream of N₂. Dry ammonia gas (35 g) was passed over thecooled sample for 1 hour. 35 g of the pretreated support were stirredfor 24 hours with 1030 ml of a 0.01 molar Pd salt solution (prepared asdescribed in Example 18). The catalyst was subsequently washed 6 timeswith 1000 ml of demineralized water and dried at 120° C. for 24 hours.The catalyst contained approx. 3% by weight of Pd.

EXAMPLE 21

Preparation of a 1% Pd-SiO₂ /Al₂ O₃ catalyst by ion exchange withPd(NH₃)₄ !²⁺ :

76.5 g of the Si/Al oxide support (15% by weight of Al₂ O₃)(Si-HP-87-069 T from Engelhard) were dehydrated for 12 hours at 400° C.in a quartz tube in a stream of N₂. Dry ammonia gas (69 g) was passedover the cooled sample for 1 hour. A 0.0033 molar Pd(NH₃)₄ !Cl₂ solutionwas prepared: 0.375 g of PdCl₂ were added to 100 ml of 0.84 molaraqueous NH₃ solution, and the mixture was stirred at 85° C. for 15minutes. After cooling, the desired molarity was established by additionof water. 35 g of the pretreated support were stirred for 24 hours with1030 ml of the 0.0033 molar Pd salt solution. The catalyst wassubsequently washed 6 times with 1000 ml of demineralized water in eachcase and dried at 120° C. for 24 hours. The catalyst contained approx.1% by weight of Pd.

EXAMPLE 22

Preparation of a 1% Pd-SiO₂ /Al₂ O₃ catalyst by treatment with PdCl₂ :

150 g of the Si/Al oxide support (15% by weight of Al₂ O₃) (Si-HP-87-069T from Engelhard) were dehydrated for 12 hours at 400° C. in a quartztube in a stream of N₂. Dry ammonia gas (60 g) was passed over thecooled sample for 1 hour. A 0.015 molar PdCl₂ solution was preparedanalogously to Example 18.35 g of the pretreated support were stirredfor 24 hours with 1000 ml of the 0.015 molar PdCl₂ solution. Thecatalyst was subsequently washed twice with 500 ml of demineralizedwater and dried at 120° C. for 24 hours. The catalyst contained approx.1.4% by weight of Pd; the chlorine content was below 0.01%.

EXAMPLE 23

Preparation of a 6% Pd-SiO₂ /Al₂ O₃ catalyst by ion exchange withPd(NH₃)₄ !²⁺ in a glass column:

900 g of the Si/Al oxide support (15% by weight of Al₂ O₃) (Si-HP-87-069T 1/8" from Engelhard) were dehydrated for 12 hours at 400° C. in aquartz tube in a stream of N₂. Dry ammonia gas (155 g) was passed overthe cooled sample for 1.25 hours. 67.6 l of a 0.01 molar Pd(NH₃)₄ !Cl₂solution were prepared: 119 g of PdCl₂ were added to 31.7 l of 0.84molar aqueous NH₃ solution, and the solution was stirred at 85° C. untilclear. After cooling, the desired molarity was established by additionof a further 35.9 l of water. The pretreated support was introduced intoa glass column (length 115 cm, diameter 6.5 cm), and the Pd solution wascirculated over the support for 15 hours by pumping (60 l/h) by means ofa peristaltic pump. The catalyst was subsequently washed 6 times in astirred vessel with 9 l of demineralized water in each case and dried at120° C. for 24 hours in an air-circulation oven. The yellow catalyst(982 g) contained approx. 6% by weight of Pd.

EXAMPLE 24

Preparation of a 6% Pd-SiO₂ /Al₂ O₃ catalyst by ion exchange of an Si/Aloxide prepared by the sol-gel process with Pd (NH₃)₄ !²⁺ :

The Si/Al oxide powder (13% by weight of Al₂ O₃) MS 13/110 from Gracewas tableted (.o slashed.9 mm). The tablets were broken, and the screenfraction from 0.315 to 1 mm was collected. 95 g of the granules weredehydrated for 12 hours at 400° C. in a quartz tube in a stream of N₂(250 ml/min). Dry ammonia gas (58 g) was passed over the quartz samplefor 1 hour. 80 g of the pretreated support were stirred for 24 hourswith 10.11 of a 0.01 molar Pd salt solution (prepared as described inExample 18). The catalyst was subsequently washed 6 times with 1000 mlof demineralized water and dried at 120° C. for 24 hours. The catalystcontained approx. 6% by weight of Pd.

EXAMPLE 25

Preparation of a 2% Pd-ZSM-5 catalyst by ion exchange with pd(NH₃)₄ !²⁺:

A pentasil zeolite (3.1% by weight of Al₂ O₃) having a particle size of0.315-1 mm contained 30% of aluminium oxide as binder. 60 g of theproduct were dehydrated for 12 hours at 400° C. in a quartz tube in astream of N₂. Dry ammonia gas (35 g) was passed over the cooled samplefor 1 hour. 20 g of the pretreated pentasil were exchanged with 420 mlof a 0.01 molar Pd salt solution (prepared as described in Example 18).The zeolite was subsequently washed 6 times with 250 ml of demineralizedwater and dried at 120° C. for 24 hours. The catalyst contained approx.2% by weight of Pd.

EXAMPLES 26-33 Table 2

Dehydrogenation of 3-methylpiperidine (MPI) to 3-picoline (PIC):

3-10 g of catalyst (particle size 0.315-1 mm) were introduced into areactor (.o slashed.13 mm). MPI was evaporated and passed over thecatalyst at the reactor temperatures indicated in Table 2 (p=1 bar). Inmost cases, a 15 ml/min stream of hydrogen was additionally established.

The product stream was analysed by gas chromatography. The analysisvalues given in Table 2 were obtained after constant reaction conditionshad become established (>20 hours):

                                      TABLE 2                                     __________________________________________________________________________    Catalyst                  T  MHSV                                                                              PIC                                                                              MPI                                       Ex.                                                                             Type      Preparation                                                                          Addition                                                                              °C.!                                                                      l/h!                                                                              GC area %!                                  __________________________________________________________________________    26                                                                              1%Pd--MgCl.sub. /Al.sub.2 O.sub.3                                                       DOS 3410542                                                                          --     270                                                                              0.25                                                                              93.6                                                                             4.3                                       27                                                                              1%Pd/Al.sub.2 O.sub.3                                                                   Example 15                                                                           15 ml/min H.sub.2                                                                    280                                                                              0.44                                                                              97.0                                                                             2.4                                       28                                                                              4%Pd/Al.sub.2 O.sub.3                                                                   Example 17                                                                           15 ml/min H.sub.2                                                                    270                                                                              0.44                                                                              98.8                                                                             1.2                                       29                                                                              5%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 18                                                                           15 ml/min H.sub.2                                                                    280                                                                              1.76                                                                              99.3                                                                             --                                        30                                                                              3%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 20                                                                           15 ml/min H.sub.2                                                                    280                                                                              1.76                                                                              99.2                                                                             0.3                                       31                                                                              1%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 21                                                                           15 ml/min H.sub.2                                                                    280                                                                              1.76                                                                              98.4                                                                             0.2                                       "-                                                                            "-                                                                            "-                                                                            "-                                                                            "-                                                                              0.88      99.0   0.2                                                        "-                                                                            "-                                                                            "-                                                                            "-                                                                              290       0.44   99.5   0.2                                                 32                                                                              1.4%Pd--  Example 22                                                                           15 ml/min H.sub.2                                                                    280                                                                              1.76                                                                              57.8                                                                             40.6                                        SiO.sub.2 /Al.sub.2 O.sub.3                                                 33                                                                              6%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 24                                                                           15 ml/min H.sub.2                                                                    280                                                                              1.76                                                                              99.3                                                                             0.3                                       "-                                                                            "-                                                                            "-                                                                            "-                                                                            "-                                                                            "-                                                                              98.4      1.2                                                               __________________________________________________________________________

It is striking that the impregnated Pd/Mg catalyst (Example 26) obtainedin accordance with an earlier patent specification (DOS 3410542) and thecatalysts obtained by impregnating aluminium oxide with Pd (Examples 27and 28) give less 3-picoline and more unreacted MPI in the productstream than do the catalysts from Examples 15-17 and 19. This is all themore surprising since the experiments with the impregnated catalystswere carried out at a low mass hourly space velocity. The catalysts ofExamples 29-31 and 33 were obtained by ion exchange of silicon/aluminiumoxide with Pd(NH₃)₄ !Cl₂. The activity can be controlled to a certainextent via the degree of exchange (cf. Examples 29-31 with 5%, and 1% ofpalladium in the exchanged catalyst). Example 32 used a catalyst inwhich the support had not been treated with Pd(NH₃)₄ !Cl₂ but insteadwith PdCl₂. This catalyst exhibited a much lower activity than thosetreated with Pd(NH₃)₄ !Cl₂.

EXAMPLES 34-40

3-10 g of catalyst (particle size 0.315-1 mm) were introduced into areactor (.o slashed.13 mm). The starting material used was a crudeproduct ("MPI crude") prepared from a mixture of the followingcomposition: 74.9% of MPI, 13.9% of 2-methyl-1,5-diaminopentane (MPDA),5.1% of organic impurities (principally methylcyclopentane diamine) and6.1% of water. The crude product was prepared by catalytic cyclizationof the MPDA obtained in the starting mixture as in Examples 15-25. Afterthe cylization, the "MPI crude" had the following composition: 89.9% ofMPI, 4.0% of organic impurities and 6.1% of water. This startingmaterial was evaporated and passed over the catalysts indicated in thetable (p≈1 bar) at the reactor temperatures shown in Table 3. In mostcases, a 15 ml/min stream of hydrogen was additionally set up. Theproduct stream was analyzed by gas chromatography.

                                      TABLE 3                                     __________________________________________________________________________    Catalyst                  T  MHSV                                                                              PIC                                                                              MPI                                       Ex.                                                                             Type      Preparation                                                                          Addition                                                                              °C.!                                                                      l/h!                                                                              GC area %!                                  __________________________________________________________________________    34                                                                              1%Pd--MgCl.sub. /Al.sub.2 O.sub.3                                                       DOS 3410542                                                                          15 ml/min H.sub.2                                                                    280                                                                              0.44                                                                              96.0                                                                             0.2                                       "-                                                                            "-                                                                            "-                                                                            "-                                                                            "-                                                                              1.76      84.5   10.1                                                       35                                                                              5%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 18                                                                           60 ml/min NH.sub.3                                                                   280                                                                              1.76                                                                              95.5                                                                             0.3                                       36                                                                              5%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                           "-                                                                              --        285    1.76   97.9                                                                             "                                                "-                                                                            "-                                                                            "-                                                                              --                                                                          "-                                                                              3.52      93.4   2.2                                                        37                                                                            "-                                                                            "-                                                                              15 ml/min H.sub.2                                                                       280    3.52   93.9                                                                             1.5                                              38                                                                              5%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 19                                                                           15 ml/min H.sub.2                                                                    280                                                                              3.52                                                                              96.0                                                                             0.4                                       39                                                                              3%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 20                                                                           15 ml/min H.sub.2                                                                    280                                                                              1.76                                                                              96.2                                                                             0.2                                       "-                                                                            "-                                                                            "-                                                                            "-                                                                              290                                                                         "-                                                                              96.5      0.3                                                               40                                                                              6%Pd--SiO.sub.2 /Al.sub.2 O.sub.3                                                       Example 24                                                                           15 ml/min H.sub.2                                                                    280                                                                              3.52                                                                              95.2                                                                             0.4                                       __________________________________________________________________________

It is striking that the impregnated Pd-Mg catalyst (Example 34) obtainedin accordance with an earlier patent specification (DOS 3410542) gives,at an MHSV of 1.76, less 3-picoline and more unreacted MPI in theproduct stream than do the catalysts from Examples 35-40. The catalystsof Examples 35-40 were obtained by ion exchange of silicon/aluminiumoxide with Pd(NH₃)₄ !Cl₂. These catalysts have a considerably higheractivity, and MPI conversions of greater than 99.5% can be achieved,even at an MHSV of 3.52.

The catalyst of Example 40 was obtained by ion exchange of asilicon/aluminium oxide prepared by the sol-gel process.

In Example 35, ammonia was metered in. The experiment shows that theammonia liberated during the cyclization of MPDA to MPI does notinterfere with the reaction. The reaction also proceeds when no hydrogencarrier gas is metered in (Example 36).

EXAMPLE 41

Pd-exchanged zeolite as catalyst:

10 g of the Pd-ZSM-5 catalyst from Example 25 (particle size 0.315-1 mm)were introduced into a reactor (.o slashed.13 mm). MPI was evaporatedand passed over the catalyst (p≈1 bar) at a reactor temperature of 280°C. and an MHSV of 0.44. The product stream was analysed by gaschromatography (GC area %). After a reaction time of 21 hours, theproduct stream contained 99.2% of PIC and 0.8% of unreacted MPI. After areaction time of 213 hours, the product stream contained 93.15% of PICand 6.85% of unreacted MPI.

EXAMPLE 42

In this experiment, it was attempted to carry out the reactionisothermally. To this end, 27 g of the catalyst from Example 19(particle size: 0.315-1 mm) were introduced into a reactor (.oslashed.21 mm). The catalyst was diluted with 53 g of the catalystsupport in such a way that the catalyst was diluted the most at thereactor inlet side, was in undiluted format the outlet side, and theconcentration gradient along the catalyst bed followed approximately anexponential function. The starting material had the followingcomposition: 92.7% of MPI, 6.5% of water, 0.8% of organic impurities.The starting material was evaporated and passed over the catalyst bed(p=0.11 bar) at an MHSV of 4.73, based on the active catalyst(corresponding to 1 g of starting material per ml of catalyst bed perhour). The product stream was analysed by gas chromatography (GC area%). The conversion was quantitative, and after 339 hours the organiccomponent of the product contained 99.3% of PIC and 0.7% of organicimpurities. Owing to the endothermicity of the reaction, a temperatureof approx. 240° C. became established in the centre of the reactor (walltemperature 280°-300° C.). The temperature at the end of the catalystbed was 300° C. over the entire reactor cross-section. After a reactiontime of 362 hours, the starting material employed was pure, anhydrousMPI. After 454 hours, the product stream contained 99.2% of PIC, 0.4% ofunreacted MPI an 0.4% of organic impurities.

EXAMPLE 43

2-Methyl-1,5-diaminopentane (MPDA) to 3-picoline continuously in 2steps:

3 g of SiO₂ /Al₂ O₃ granules (Si-HP-87-069 T from Engelhard) in aparticle size of 0.315-1 mm were introduced into a reactor (.oslashed.13 mm). MPDA was evaporated and passed over the catalyst with a15 ml/min carrier-gas stream of H₂ at a pressure of approx. 1 bar end areactor temperature of 320° C., and was cyclized to MPI. The MPDA usedwas a commercial product obtainable from Du Pont de Nemours under thetrade name Dytek A. The product from the cyclization reactor was kept inthe gas phase and passed directly to a second reactor. This reactorcontained 3 g of the dehydrogenation catalyst from Example 18 (particlesize 0.32-1 mm). The reactor temperature was 280° C. and the pressurewas 1 bar. During the experiment, the starting material was changed fromMPDA to MPI and then to a crude product (3-MP crude) comprising amixture having the following composition: 74.9% of MPI, 13.9% of MPDA,5.1% of organic impurities (principally methylcyclopentanediamines) and6.1% of water. The results with the corresponding MHSVs (MHSV based onreactor 1) are shown in Table 4 below:

Table 4:

    ______________________________________                                        Starting                                                                              MHSV     PIC     MPI   Run time                                                                              Deactivation                           material                                                                               l/h!     GC area %!  h!      PIC %/h!                                ______________________________________                                        Dytek A 2.1      99.7    --    71      0                                      "-      3.15     99.6    0.2   25      0                                      "-      4.2      98.6    1.4   48      0                                      MPI     4.1      95.2    3.8    3      --                                     "-      3.52     98.6    0.6   92      0                                      3-MP crude                                                                            4.2      93.9    1.5   170     0.0172                                 ______________________________________                                    

EXAMPLE 44

2-Methyl-1,5-diaminopentane (MPDA) to 3-picoline continuously in 2steps:

3 g of SiO₂ /Al₂ O₃ granules (Si-HP-87-069 T from Engelhard) in aparticle size of 0.315-1 mm were introduced into a reactor (.oslashed.13 mm). MPDA was evaporated and passed over the catalyst with a15 ml/min carrier-gas stream of H₂ at a pressure of approx. 1 bar and areactor temperature of 320° C., and was cyclized to MPI. The MPDA usedwas a commercial product obtainable from Du Pont de Nemours under thetrade name Dytek A. The product from the cyclization reactor was kept inthe gas phase and passed directly to a second reactor. This reactorcontained 3 g of the dehydrogenation catalyst from Example 20 (particlesize 0.315-1 mm). The reactor temperature was 280° C. and the pressurewas 1 bar. During the experiment, the starting material was changed fromMPDA to a crude product (3-MP crude) comprising a mixture having thefollowing composition: 74.9% of MPI, 13.9% of MPDA, 5.1% of organicimpurities (principally methyl-cyclopentanediamines) and 6.1% of water.The results with the corresponding MHSVs (MHSV based on reactor 1) areshown in Table 5 below:

                  TABLE 5                                                         ______________________________________                                        Starting                                                                              MHSV     PIC     MPI   Run time                                                                              Deactivation                           material                                                                               l/h!     GC area %!  h!      PIC %/h!                                ______________________________________                                        Dytek A 2.1      97.5    1.4   117     0.0204                                 "-      1.0      98.2    0.7    18     0                                      3-MP crude                                                                    "-      97.6     0.2     119   0.0248                                         ______________________________________                                    

EXAMPLE 45

3-MP crude to 3-picoline, continuously in 2 steps with intermediate tarseparator:

Compared with Example 44, the starting material had a differentcomposition and a tar separator was installed between the 1st and 2ndreactors. 3 g of SiO₂ /Al₂ O₃ granules (Si-HP-87-069 T from Engelhard)having a particle size of 0.315-1 mm were introduced into a reactor (.oslashed.13 mm). The starting material was a crude product (3-MP crude)having the following composition: 45.8% of MPI, 29.9% of MPDA, 9.8% oforganic impurities (principally methylcyclopentane-diamines) and 14.5%of water. The starting material was evaporated and passed through thereactor with a 15 ml/min carrier-gas stream of H₂ at an MHSV of 4.2 andat a pressure of approx. 1 bar and a reactor temperature of 320° C. Theproduct from the cyclization reactor was passed through a tar separator(115° C.) and fed directly to a second reactor. This reactor contained 3g of the dehydrogenation catalyst from Example 23 (particle size 0.315-1mm). The reactor temperature was 280° C. After a reaction time of 335hours, the organic phase of the product contained 94.6% of PIC and 5.4%of organic impurities (GC area %) with quantitative conversion of MPDAand MPI. Catalyst deactivation was not observed.

We claim:
 1. A process for the preparation of 3-methylpiperidine from2-methyl-1,5-diaminopentane in the gas phase, comprising passing gaseous2-methyl-1,5-diaminopentane at a temperature of 300° to 400° C. and apressure of from 0 to 10 bar above atmospheric pressure without additionof ammonia over a catalyst which is selected from the group consistingof activated Al₂ O₃, an aluminum/silicon mixed oxide or a natural orsynthetic zeolite, has a ratio between acidic centers and basic centerson the surface of greater than 2 and has a specific surface area ofgreater than 40 m² /g, whereby 3-methylpiperidine is produced in a yieldof at least 89 percent.
 2. The process according to claim 1, wherein thereaction is conducted at a temperature of 305° to 375° C.
 3. The processaccording to claim 2, wherein the catalyst is an aluminum/silicon mixedoxide.
 4. A process for the preparation of 3-methylpyridine comprising(a) passing gaseous 2-methyl-1,5-diaminopentane at a temperature of 300°to 400° C. and a pressure of from 0 to 10 bar above atmospheric pressurewithout addition of ammonia over a catalyst whereby 3-methylpyridine isproduced, said catalyst being selected from the group consisting ofactivated Al₂ O₃, an aluminum/silicon mixed oxide or a natural orsynthetic zeolite, has a ratio between acidic centers and basic centerson the surface of greater than 2 and has a specific surface area ofgreater than 40 m² /g, whereby 3-methylpiperidine is produced in a yieldof at least 89 percent, and (b) subsequently passing the3-methylpiperidine over a second catalyst which is a dehydrogenationcatalyst, whereby the 3-methylpyridine is produced.
 5. The processaccording to claim 4, wherein the reaction in step (a) is conducted at atemperature of 305° to 375° C.
 6. The process according to claim 5,wherein the catalyst in step (a) is an aluminum/silicon mixed oxide. 7.The process according to claim 4, wherein the dehydrogenation catalystis a noble metal on a support.
 8. The process according to claim 4,wherein the dehydrogenation is carried out at 220° to 400° C.
 9. Theprocess according to claim 8, wherein the dehydrogenation catalyst is anoble metal on a support.
 10. The process according to claim 9, whereinthe noble metal used is palladium or platinum.
 11. Process according toclaim 10, wherein the dehydrogenation catalyst used is palladium on anamorphous silicon/aluminum oxide prepared by ion exchange with a solublepalladium complex.
 12. A process for the preparation of3-methylpiperidine from 2-methyl-1,5-diaminopentane in the gas phase,comprising passing gaseous 2-methyl-1,5-diaminopentane at a temperatureof 300° to 400° C. and a pressure of from 0 to 10 bar above atmosphericpressure without addition of ammonia over a catalyst which is selectedfrom the group consisting of activated Al₂ O₃, a natural zeolite and asynthetic zeolite, has a ratio between acidic centers and basic centerson the surface of greater than 2 and has a specific surface area ofgreater than 40 m² /g.
 13. A process for the preparation of3-methylpyridine comprising (a) passing gaseous2-methyl-1,5-diaminopentane at a temperature of 300° to 400° C. and apressure of from 0 to 10 bar above atmospheric pressure without additionof ammonia over a catalyst whereby 3-methylpyridine is produced, saidcatalyst being selected from the group consisting of activated Al₂ O₃, anatural zeolite and a synthetic zeolite, has a ratio between acidiccenters and basic centers on the surface of greater than 2 and has aspecific surface area of greater than 40 m² /g, and (b) subsequentlypassing the 3-methylpiperidine over a second catalyst which is adehydrogenation catalyst, whereby the 3-methylpyridine is produced.